1. Field of the Invention
The present invention relates to a light source device for generating extreme ultra violet (EUV) light by applying a laser beam to a target. Furthermore, the present invention relates to exposure equipment using such a light source device.
2. Description of a Related Art
With finer semiconductor processes, the photolithography makes rapid progress to finer fabrication, and, in the next generation, microfabrication of 100 to 70 nm, further, microfabrication of 50 nm or less will be required. For example, in order to fulfill the requirement for microfabrication of 50 nm or less, the development of exposure equipment with a combination of an EUV light source of about 13 nm in wavelength and a reduced projection catoptric system is expected.
As the EUV light source, there are three kinds of light sources, which include an LPP (laser produced plasma) light source using plasma generated by applying a laser beam to a target, a DPP (discharge produced plasma) light source using plasma generated by discharge, and an SR (synchrotron radiation) light source using orbital radiation. Among them, the LPP light source has advantages that extremely high intensity near black body radiation can be obtained because plasma density can be considerably made larger, light emission of only the necessary waveband can be performed by selecting the target material, and there is no structure such as electrodes surrounding the light source and an extremely large collection solid angle of 2π sterad can be ensured because it is a point source having substantially isotropic angle distribution. Therefore, the LPP light source is thought to be predominant as a light source for EUV lithography requiring power of several tens of watts.
In the LPP light source, in the case where a solid material is used as a target to which a laser beam is applied for generating plasma, the heat generated by the laser beam application is conducted to the periphery of the laser beam applied region when the laser beam applied region turns into a plasma state, and the solid material is melted on the periphery thereof. The melted solid material radiates in large quantity as debris of more than several micrometers in diameter, and that causes damage to the collector mirror (specifically, to the mirror coating) to reduce the reflectance thereof. On the other hand, in the case where a gas is used as the target, although an amount of debris is reduced, the conversion efficiency from the power supplied to the driving laser into the power of EUV light is reduced.
A conventional light source device is shown in FIG. 14. A material as a target is injected downwardly from a nozzle 101. Plasma 104 is generated by applying a laser beam formed by converging laser light generated from a driving laser 102 with a focusing lens 103. The EUV light radiating from the plasma 104 is collected by the collector mirror 105, passes through a debris shield 107 as luminous flux (e.g., parallel light) 106, and then, transmitted to an exposure device.
At that time, in order to suppress the damage provided to the collector mirror 105 by the debris radiating with the EUV light, the collector mirror 105 is required to be separated from the region in which the laser beam is applied to the target. Further, in order to maintain the collection rate of EUV light, as the distance between the laser beam applied region and the collector mirror 105 becomes longer, the size of the collector mirror 105 is required to be made larger.
As a related technology, Japanese Patent Application Publication JP-B-3433151 discloses a laser plasma X-ray source in which damage to an optical mirror due to generated debris is prevented and the collection efficiency of X-ray is improved. This laser plasma X-ray source includes a magnetic field applying device for applying a magnetic field in a direction perpendicular to an injection direction of a target. Assuming that the traveling direction of the debris before deflected by the magnetic field is the injection direction of the target, by locating the optical mirror in a direction in which ionic state debris deflected by the magnetic field does not fly, the damage to the optical mirror can be prevented.
However, the fact is that the debris radiating from plasma flies in almost all directions. Further, since the ions emitted from the plasma have energy of several kilo electron volts, the traveling velocity thereof reaches several tens of kilometers per second. Therefore, assuming that the traveling velocity of plasma is several hundreds of meters per second that is the same as the traveling velocity of the target before becoming plasma, it is not effective to try to change the traveling direction of debris by the magnetic field, and the debris flies in almost all directions and the damage to the optical mirror can not be prevented.
In this document, since the necessary magnetic flux density is on the order from 10−1T to 100T at the highest, the purpose can be achieved by using a commercially available strong permanent magnet. However, in order to generate such magnetic flux density, the distance between the permanent magnet and the laser beam applied region is required to be made very short. When the distance between the permanent magnet and the laser beam applied region is made very short, there is a problem that the collection solid angle of the optical mirror is significantly limited.
On the other hand, Japanese Patent Application Publication JP-B-2552433 discloses a removing method and device capable of radically removing debris generated from a solid target in a relative simple manner. According to the document, electric charges are provided to neutral fine particles produced with X-rays from plasma on the surface of a target material, an electromagnetic field in which an electric field and a magnetic field are mutually perpendicular by a pair of mesh-form electrodes arranged along the pathway of X-ray and an electromagnet disposed between the pair of electrodes, the charged fine particles are passed through the electromagnetic field, and thus, the orbit of the charged fine particles can be bend and eliminated to the outside of the X-ray pathway. Thereby, an X-ray optical element provided on the X-ray pathway can be protected.
However, in the case where the method is applied to the conventional light source device as shown in FIG. 14, it is necessary to dispose the electromagnet and electrodes between the generated plasma 104 and collector mirror 105. Accordingly, the EUV light is shielded by the electromagnet and electrodes and the collection solid angle becomes very small because of the long distance between the laser beam applied region and the collector mirror 105. Therefore, there is a problem that the collection rate of EUV light is drastically reduced.